Metal particle

ABSTRACT

According to this invention, provided is a metal particle that includes an intermetallic compound composed of Sn, Cu and Ni, in a basal phase that contains Sn and an Sn—Cu alloy, and at least parts of the Sn—Cu alloy and the intermetallic compound in the basal phase form an endotaxial joint.

INCORPORATION BY REFERENCE

This application is based on Japanese Patent Application No.2019-154628, filed on Aug. 27, 2019, the content of which isincorporated hereinto by reference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

This invention relates to a metal particle.

2. Description of the Related Art

In an advancing situation of IoT (Internet of Things) with ever-growingneeds for energy saving, power semiconductor that holds the key for thetechnology has been increasingly gaining its importance, while leavingmany problems on effective use thereof to be solved. The powersemiconductor, which handles high power ascribed to high voltage andlarge current, produces large amount of heat and becomes hot. Si powersemiconductor, although having been suited to a required level of heatresistance of approximately 175° C. at present, are on the way todevelopment towards a Si power semiconductor durable to highertemperatures, approximately at 200° C. Next-generation powersemiconductors with use of SiC and GaN, for example, are even requiredto endure 250 to 500° C.

The best way to enhance the heat resistance might rely upon a techniqueof joining devices and components onto a Cu substrate that excels inheat dissipation. Difference of coefficients of thermal expansion amongthe materials would, however, result in breakage of the devices andcomponents, or fracture of a joint material at a joint area. Thus anexpensive ceramic substrate whose coefficient of thermal expansion isclose to those of the devices and components has been used at present,the situation needing further improvement.

Referring now to the joint material, there has been no joint materialwhich can attain a high level of heat resistance required for thenext-generation power semiconductor with use of SiC or GaN.

For example, an SnAgCu-based joint material (powdery solder material)disclosed in JP-A-2007-268569 is no more than a material applicable tothe power semiconductor of a class of approximately 125° C., and is notapplicable to the next-generation power semiconductor.

For full demonstration of performances of the power semiconductor, it isnecessary to avoid the aforementioned breakage of the devices andcomponents, or fracture of the joint material in the joint area, evenwhen materials with different coefficients of thermal expansion arejoined. If a joint material having such high heat resistance and highreliability, and free of environmental pollutant such as lead weresuccessfully put into practical use for the power semiconductor, powerelectronics industry making use of the power semiconductor would growdramatically.

On the other hand, the present applicant has proposed in Japanese PatentNo. 6029222 a metal particle that includes a shell and a core, whereinthe core contains a metal or an alloy, the shell contains a meshstructure of an intermetallic compound and covers the core, meanwhilethe core contains Sn or Sn alloy, and the shell contains anintermetallic compound of Sn and Cu. The joint area formed of this metalparticle has been proven to retain high levels of heat resistance, jointstrength and mechanical strength over a long period, even afterprolonged operation at high temperatures, or after used in a harshenvironment typically involving large temperature change from hightemperature operational state down to low temperature idle state.

The metal particle disclosed in Japanese Patent No. 6029222 has a doublelayered structure with the shell and the core, wherein placement of theintermetallic compound in the shell, between a substrate and an objectto be joined, contributes to suppress Cu and so forth from diffusinginto the object to be joined, to thereby successfully suppressKirkendall void from generating. The metal particle has, however, notyet reached reliable mutual joining of the devices and the componentshaving different coefficients of thermal expansion.

CITATION LIST

-   [Patent Document 1] JP-A-2007-268569-   [Patent Document 2] Japanese Patent No. 6029222

SUMMARY OF THE INVENTION

It is therefore an object of this invention to provide a metal particlehaving heat resistance and joint strength higher than those in the priorart, and is capable of mutually joining any devices and componentshaving different coefficients of thermal expansion in a reliable manner.

The present inventor went through extensive investigations, and foundthat aforementioned problem can be solved by using a metal particle thatcontains a specific intermetallic compound in a specific basal phase,and at least parts of the basal phase and the intermetallic compoundform an endotaxial joint. The finding led the present inventor to arriveat this invention.

That is, this invention is summarized as follows.

1. A metal particle comprising an intermetallic compound composed of Sn,Cu and Ni, in a basal phase that contains Sn and an Sn—Cu alloy, atleast parts of the Sn—Cu alloy and the intermetallic compound in thebasal phase forming an endotaxial joint.

2. The metal particle according to 1, including 0.7 to 40% by mass ofCu, 0.1 to 5% by mass of Ni, and the balance of Sn.

3. The metal particle according to 1 or 2, having a particle size of 1μm to 50 μm.

Advantageous Effects of Invention

Sn has a tetragonal crystal structure within a temperature range fromapproximately 13° C. to approximately 160° C. (Sn with the tetragonalcrystal structure will be referred to as β-Sn), which causes transitionto a cubic crystal structure in a lower temperature region (Sn with thecubic crystal structure will be referred to as α-Sn). The β-Sn crystalstructure also causes transition to a high temperature phase crystalcalled orthorhombic crystal structure in a temperature region aboveapproximately 160° C. (Sn with the orthorhombic crystal structure willbe referred to as γ-Sn). The phase transition between the tetragonalβ-Sn and cubic α-Sn is known to be accompanied by a particularly largevolumetric change.

The metal particle of this invention is featured by that it contains ahigh temperature phase crystal even at approximately 160° C. or below(at room temperature, for example). For example, if the joint materialthat contains the metal particle is heated during the joining process,so as to bring the joint material into a semi-molten state rather thanin full molten state, to thereby create a state containing theendotaxial joint formed between the intermetallic compound and the basalphase, the joint structure will retain a state that contains the hightemperature phase crystal even after cooled down into a temperaturerange of 160° C. or below. Such high temperature phase crystal is lesslikely to cause phase transition to the tetragonal low temperature β-Snphase, even if the temperature is lowered down to a certain degree. Snthus remained unchanged, without causing phase transition into thetetragonal β-Sn, will not cause phase transition into α-Sn, andtherefore does not cause a large volumetric change in association withthe phase transition into α-Sn under lowered temperature. Hence, even ina temperature range of 160° C. or below (at room temperature, forexample), the joint material that contains Sn with the high temperaturephase crystal is relieved from a large volumetric change due totemperature change, as compared with other joint material that containsSn in its chemical composition (that is, Sn having no high temperaturephase crystal intentionally contained therein, which is retainable evenin a temperature range of 160° C. or below).

The joint structure formed by using the metal particle of this inventionretains the endotaxial joint in the metal particle, and preferablyretains self-similar (fractal) crystal structure resulted from theendotaxial joint, proving possibility of providing a high level of heatresistance required for the next-generation power semiconductor.

Electronic components employ various metals including Cu, Ag, Au, Ni andso forth, with which Sn can join in a reliable manner.

With the high temperature phase crystal contained therein over a widetemperature range (even at room temperature, for example), and with thetetragonal low temperature β-Sn phase suppressed from generating thereinas possible, the metal particle of this invention is featured by itsunlikeliness of causing a large volumetric change in association withphase transition from tetragonal β-Sn to cubic α-Sn under temperaturechange, and can join with various metals employed in electroniccomponents. Hence, the metal particle is especially beneficial tojoining of minute joint part.

As described above, this invention can provide a metal particle capableof forming a joint in which the volumetric change is suppressed over atemperature range wider than in the prior art, having heat resistance,joint strength and mechanical strength whose levels are higher thanthose in the prior art, and capable of mutually joining the devices andthe components having different coefficients of thermal expansion.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a STEM image of a cross section of a metal particle of thisinvention thinned with focused ion beam (FIB).

FIG. 2 is a schematic drawing illustrating an exemplary manufacturingapparatus suitably applicable to manufacture of the metal particle ofthis invention.

FIG. 3 illustrates results of EDS element mapping obtained from thecross section of the metal particle illustrated in FIG. 1 .

FIG. 4 summarizes quantified values of Cu, Ni and Sn at various pointson the cross section of the metal particle illustrated in FIG. 1 .

FIGS. 5A and 5B are TEM images of a cross section of the metal particleobtained in Example 1, and FIG. 5C is a transmission electrondiffraction pattern of the metal particle.

FIG. 6 presents optical microphotographs of a cross section of a jointarea formed by joining a copper substrate and a silicon device using ajoint material that contains the metal particle obtained in Example 1,and then by subjecting them to a thermal shock test.

FIG. 7A is an STEM image of a cross section of an SnAgCu-based jointmaterial in the prior art, and FIGS. 7B to 7D illustrate results of EDSelement mapping.

FIG. 8 presents optical microphotographs of a cross section of a jointarea formed by joining a copper substrate and a silicon device using ajoint material that contains the metal particle obtained in ComparativeExample 1, and then by subjecting them to a thermal shock test.

DESCRIPTION OF THE EMBODIMENTS

This invention will further be detailed below.

Terminology in this patent specification will be defined as follows,unless otherwise specifically noted.

(1) The term “metal” is used not only to encompass metal element as asimple substance, but also occasionally to encompass alloy andintermetallic compound composed of two or more metal elements.

(2) When referring to a certain metal element as a simple substance, itmeans not only an absolutely pure substance solely composed of suchmetal element, but also a substance containing a trace amount of othersubstance. That is, the metal element of course does not mean to excludea case where a trace impurity that hardly affects properties of thatmetal element is contained. As for the basal phase for example, it doesnot mean to exclude a case where a part of atoms in Sn crystal isreplaced by other element (Cu, for example). For example, such othersubstance or other element may account for 0 to 0.1% by mass of themetal particle.

(3) Endotaxial joint means that an intermetallic compound precipitatesin a substance which is expected to become metal or alloy (in thisinvention, the basal phase that contains Sn and an Sn—Cu alloy), whereinthe Sn—Cu alloy and the intermetallic compound join during theprecipitation while attaining lattice matching, to thereby producecrystal grains. The term “endotaxial” is a known term, which is foundfor example in the last paragraph on the left column on page 160, inNature Chemistry, 3(2): 160-6, 2011.

A metal particle of this invention is featured by having anintermetallic compound composed of Sn, Cu and Ni, in a basal phase thatcontains Sn and an Sn—Cu alloy, and at least parts of the Sn—Cu alloyand the intermetallic compound in the basal phase form an endotaxialjoint.

FIG. 1 is a STEM image of a cross section of the metal particle of thisinvention thinned with focused ion beam (FIB). Particle size of themetal particle of this invention, which is approximately 5 μm in FIG. 1, is preferably within the range from 1 μm to 50 μm for example.Referring now to the metal particle in FIG. 1 , observed are a basalphase 140 that contains Sn and an Sn—Cu alloy, and resides therein anintermetallic compound 120 that is composed of Sn, Cu and Ni. Theintermetallic compound 120 is confirmed to have a self-similar (fractal)crystal structure.

The metal particle of this invention typically contains 0.7 to 40% bymass of Cu, 0.1 to 5% by mass of Ni, and the balance of Sn; andpreferably contains 1 to 15% by mass of Cu, 1 to 3% by mass of Ni, andthe balance of Sn.

The metal particle of this invention may be manufactured typically froma starting material having a chemical composition represented by 8% bymass of Cu, 1% by mass of Ni and 91% by mass Sn (referred to as8Cu.91Sn.1Ni, hereinafter). For example, the metal particle isobtainable by melting 8Cu.91Sn.1Ni at approximately 650° C., feeding themolten material onto a dish-like disk which is kept spinning at highspeed in a nitrogen atmosphere, so as to centrifugally scatter themolten metal in the form of fine droplets, and by cooling andsolidifying the droplets under reduced pressure.

A preferred example of a manufacturing apparatus suitable formanufacture of the metal particle of this invention will be explainedreferring to FIG. 2 . A granulation chamber 1 has a cylindrical top anda conical bottom, and has a lid 2 placed on the top. The lid 2 has anozzle 3 perpendicularly inserted at the center thereof, and right underthe nozzle 3 arranged is a dish-like rotating disk 4. Reference sign 5represents a mechanism that supports the dish-like rotating disk 4 so asto be movable up and down. At the lower end of the conical bottom of thegranulation chamber 1, connected is a discharge pipe 6 through which theproduced particles are output. An upper end of the nozzle 3 is connectedto an electric furnace (high frequency induction furnace) 7 in which ametal to be granulated is melted. An atmospheric gas, having thechemical composition specifically adjusted in a mixed gas tank 8, is fedthrough a pipe 9 and a pipe 10, respectively into the granulationchamber 1 and to the top of the electric furnace 7. Inner pressure ofthe granulation chamber 1 is controlled by a valve 11 and a ventilator12, and inner pressure of the electric furnace 7 is controlled by avalve 13 and a ventilator 14. The molten metal fed through the nozzle 3onto the dish-like rotating disk 4 is scattered in the form of finedroplets with the aid of centrifugal force of the dish-like rotatingdisk 4, and then solidified after cooled under reduced pressure. Thethus produced solid particles are fed through the discharge pipe 6 to anautomatic filter 15, where the particles are classified. Reference sign16 represents a particle collector.

A process of bringing the molten metal from the hot molten state down tothe cold solidified state is the key for formation of the metal particleof this invention.

The process is carried out under conditions exemplified below.

With the melting temperature of metal in the electric furnace 7 presetto 600° C. to 800° C., the molten metal kept at that temperature is fedthrough the nozzle 3 onto the dish-like rotating disk 4.

The dish-like rotating disk 4 is a dish-like disk having an innerdiameter of 60 mm and a depth of 3 mm, which is rotated at 80,000 to100,000 rpm.

A vacuum chamber which can be evacuated down to 9×10⁻² Pa or around isemployed here as the granulation chamber 1, and is evacuated, to whichnitrogen gas conditioned at 15 to 50° C. is fed while concurrentlyventilating the chamber, so as to adjust the pressure in the granulationchamber 1 to 1×10⁻¹ Pa or below.

The metal particle manufactured under such conditions will have aparticle size preferably within the range from 1 μm to 50 μm asdescribed above, which is more preferably 5 μm to 40 μm.

Chemical composition of the intermetallic compound in the metal particleof this invention, when expressed in terms of proportion of numbers ofSn, Cu and Ni atoms, is given typically by Sn:Cu:Ni=(40 to 60):(30 to50):(4 to 9).

The intermetallic compound in the metal particle of this inventiontypically accounts for 20 to 60% by mass of the whole metal particle,wherein the percentage is more preferably 30 to 40% by mass.

The chemical composition and the percentage of the intermetalliccompound may be satisfied by following the aforementioned conditions formanufacturing the metal particle.

The metal particle of this invention can be formed into a sheet orpaste, bringing it into contact with an object to be joined, holdingthem at 160° C. to 180° C. for 3 minutes or longer, then by allowing thesheet or paste to melt at 235° C. to 265° C., and allowed to solidify. Agood joint structure can thus be formed.

The sheet that contains, as a material, the metal particle of thisinvention is obtainable typically by compressing the metal particlebetween rollers, typically as described below. That is, the metalparticle of this invention is fed between a pair of pressure rollersthat rotate in opposing directions, and then compressed while beingheated through the pressure rollers to approximately 100° C. to 150° C.

The metal particle of this invention is alternatively obtainable in theform of conductive paste, by allowing it to disperse in an organicvehicle.

The sheet or the conductive paste may be formed of a mixture of metalparticle, by adding other particle such as SnAgCu-based alloy particle,Cu particle, Cu alloy particle, Ni particle, Ni alloy particle, ormixture of any of these particles. Such other particle may optionally becoated with a metal such as Si.

For example, by combining the metal particle with Cu particle or Nialloy particle which is more conductive than Sn, obtainable is a metaljoint layer which is highly conductive, and whose volumetric change issuppressed over a relatively wide temperature range.

EXAMPLES

This invention will further be explained below referring to Examples andComparative Examples. This invention is, however, not limited toExamples below.

Example 1

A metal particle 1 having a diameter of approximately 3 to 40 μm wasmanufactured from 8Cu.91Sn.1Ni as a starting material, with use of themanufacturing apparatus illustrated in FIG. 2 .

Conditions below were employed for the process.

A melting crucible was placed in the electric furnace 7, into which8Cu.91Sn.1Ni was placed and melted at 650° C., and while keeping thetemperature, the molten metal was fed through the nozzle 3 onto thedish-like rotating disk 4.

The dish-like rotating disk 4 employed here was a dish-like disk with aninner diameter of 60 mm and a depth of 3 mm, which was rotated at 80,000to 100,000 rpm.

The granulation chamber 1 which can be evacuated down to around 9×10⁻²Pa was evacuated, to which nitrogen gas at 15 to 50° C. was fed andconcurrently evacuated, to thereby adjust the inner pressure of thegranulation chamber 1 to 1×10⁻¹ Pa or below.

The obtained metal particle 1 was found to have a cross sectionpresented in FIG. 1 .

FIG. 3 illustrates results of EDS element mapping obtained from thecross section of the metal particle illustrated in FIG. 1 . From theanalytical results, the metal particle was found to be composed of10.24% by mass of Cu, 0.99% by mass of Ni, and 88.76% by mass of Sn asthe balance.

The intermetallic compound in the metal particle 1 was found to accountfor 30 to 35% by mass of the metal particle.

FIG. 4 summarizes quantified values of Cu, Ni and Sn at various pointson the cross section of the metal particle 1 illustrated in FIG. 1 .

As summarized in FIG. 4 , individual points pt1 to pt7 on the crosssection of the metal particle were found to give different quantifiedvalues of Cu, Ni and Sn.

This indicates that the intermetallic compound forms a fractal crystalstructure in the basal phase metal.

FIGS. 5A and 5B are TEM images of the cross section of the metalparticle 1, and FIG. 5C is a transmission electron diffraction patternof the metal particle 1.

Referring now to FIG. 5A, the intermetallic compound 120 composed of Sn,Cu and Ni is found to reside in the basal phase 140 that contains Sn andan Sn—Cu alloy.

FIG. 5B is an enlarged view of a part surrounded by a rectangular framein FIG. 5A. Referring now to FIG. 5B, lattice constants (and crystalorientations) agree between the basal phase 140 and the intermetalliccompound 120 (0.30 nm in FIG. 5B), proving that the individual crystalsare joined while attaining lattice matching. That is, the endotaxialjoint was confirmed from FIG. 5B indicating joining of the lattices, andalso absence of a buffer layer between the crystals was confirmed fromFIG. 5(C) that illustrates the transmission electron diffraction patternof the interface between the basal phase 140 and the intermetalliccompound 120.

In the metal particle of this invention, area ratio of the endotaxialjoint, when assuming the total area of joint face between the basalphase and the intermetallic compound as 100%, is preferably 30% orlarger, and more preferably 60% or larger. The area ratio of theendotaxial joint may be calculated typically as follows.

A cross section of the metal particle, such as presented in FIG. 1 , isphotographed under an electron microscope, and joint faces between theintermetallic compound and the Sn—Cu alloy are sampled at 50 freelyselected points. The joint faces are then examined by image analysis, tothereby determine to what degree the endotaxial joint, such as presentedin FIG. 5 , resides in the sampled joint faces.

It was also found from FIGS. 5A to 5C that at least a part of Sn in themetal particle of this Example retains the high temperature phasecrystal even at normal temperature.

It was also found from FIGS. 5A to 5C that the interface of theendotaxial joint has a fractal crystal structure. With the fractalcrystal structure contained therein, the intermetallic compound canovercome its brittleness, the high temperature phase crystal of Sn willbecome more likely to be retained, devices and components havingdifferent coefficients of thermal expansion will be mutually joined in amore reliable manner, and the joint structure will be kept joinedreliably, even exposed to high/extremely-low temperature cycles.

Next, the metal particle 1 in a dry powder form was compressed tomanufacture a sheet, the sheet was then used for joining a coppersubstrate and a silicon device, and subjected to a high temperaturestorage test (HTS) at 260° C. Results indicated that shear strengthelevated from approximately 50 MPa up to approximately 60 MPa, over aperiod ranging from the start of test until approximately 100 hoursafter, and remained plateau at approximately 60 MPa over a temporalrange beyond 100 hours.

On the other hand, temperature cycle test (TCT) ranged from −40 to 200°C. yielded results indicating that the shear strength was stabilized atapproximately 50 MPa over the whole cycles (1000 cycles).

FIG. 6 presents optical microphotographs of the cross section of thejoint area formed by joining the copper substrate and the silicon devicewith use of the joint material that contains the metal particle 1, andthen by subjecting them to a thermal shock test.

The thermal shock test was conducted 1000 cycles at a lower exposuretemperature of −40° C. and a higher exposure temperature of 175° C.

As can be understood from FIG. 6 , a good state of joint was retained,without causing decay of the joint area between the copper substrate andthe silicon device, and without causing fracture of the silicon device.

Example 2

A metal particle 2 was manufactured in the same way as in Example 1,except by using a starting material composed of 8% by mass of Cu, 3% bymass of Ni and 89% by mass of Sn.

Next, 70% by mass of the metal particle 2, and 30% by mass of alloypowder composed of 90% by mass of Cu and 10% by mass of Ni werehomogeneously mixed, and the mixture in a dry powder form was compressedto manufacture a sheet (50 μm thick). The sheet was then used forjoining the copper substrate and the silicon device, and subjected tothe high temperature storage test (HTS) at 260° C. Results indicatedthat shear strength elevated from approximately 60 MPa up toapproximately 70 MPa, over a period ranging from the start of test untilapproximately 100 hours after, and remained plateau at approximately 60MPa over a temporal range beyond 100 hours.

On the other hand, temperature cycle test (TCT) ranged from −40 to 200°C. yielded results indicating that the shear strength was stabilized atapproximately 50 MPa over the whole cycles (1000 cycles).

Comparative Example 1

As a comparative example, FIG. 7A presents an STEM image of a crosssection of an SnAgCu-based joint material (powdery solder material witha particle size of 5 μm) in the prior art, and FIGS. 7B to 7D illustrateresults of EDS element mapping.

It was confirmed from FIGS. 7A to 7D that the prior SnAgCu-based jointmaterial does not contain the intermetallic compound, instead having asingle metal element dispersed therein. It was also confirmed that theSn—Cu alloy of the metal basal phase does not have the high temperaturephase crystal structure. Such prior SnAgCu-based joint material causeddecay in the joint area in the temperature cycle test (TCT) ranged from−40 to 200° C., before going through 100 cycles, which was far fromachieving heat resistance and strength comparable to those of the metalparticle of this invention.

FIG. 8 presents optical microphotographs of the cross section of thejoint area formed by joining the copper substrate and the silicon deviceusing the joint material obtained in Comparative Example 1, and then bysubjecting them to the thermal shock test.

The thermal shock test was conducted 50 cycles at a lower exposuretemperature of −40° C. and a higher exposure temperature of 175° C.

As can be confirmed from FIG. 8 , the joint area between the coppersubstrate and the silicon device was found to decay as early as the 50thcycle after the start of the thermal shock test.

Having detailed this invention referring to the attached drawings, thisinvention is not limited to these Examples. It is apparent that thoseskilled in the art will easily arrive at various modifications, on thebasis of basic technical spirit and teaching of this invention.

What is claimed is:
 1. A metal particle comprising an intermetalliccompound composed of Sn, Cu and Ni, in a basal phase that contains Snand an Sn—Cu alloy, at least parts of the Sn—Cu alloy and theintermetallic compound in the basal phase forming an endotaxial joint.2. The metal particle according to claim 1, having a particle size of 1μm to 50 μm.
 3. The metal particle according to claim 1, comprising 0.7to 40% by mass of Cu, 0.1 to 5% by mass of Ni, and the balance of Sn. 4.The metal particle according to claim 3, having a particle size of 1 μmto 50 μm.